The present invention relates to a method for operating a position measuring system and to a suitable position measuring system for that purpose.
Position measuring systems for determining the relative and absolute positions of two parts that are movable relative to one another are known. These systems include a scale having an incremental graduation as well as one or more reference marks for determining the absolute positional relationship between the two parts. Until the first or second time a reference mark is scanned, incremental scanning signals are generated by a scanning unit of the position measuring system as it scans the incremental graduation of the scale and are delivered to a counter unit. From the counter unit, incremental position data are transmitted in serial form to an evaluation unit located downstream. Thus, at defined sampling times, the current counter value of the counter unit is determined by the position measuring system, and the incremental position data are transmitted in the form of a code word to the evaluation unit. As soon as the reference marks on the scale are scanned by the scanning unit for the first or second time, the exact absolute position of the two parts is known. After that it is possible to transmit absolute position data or absolute position signals to the evaluation unit in the form of code words at the predetermined sampling times. The absolute relationship can be established by a single reference mark with a known position, or by a plurality of so-called spacing-encoded reference marks.
A problem with these systems arises, however, if the transmitted serial position data are then needed by the evaluation unit for the purpose of speed control. The problem arises with regard to determining the exact speed at the transition from incremental to absolute position measurement. In particular, the first time absolute position data are transmitted in the form of a code word that indicates the exact absolute position, a correct determination of the relative speed of the two parts in the preceding interval.
An object of the present invention is to disclose a method for operating a position measuring system and a suitable position measuring system with which position data can be transmitted in serial form to a downstream evaluation unit. In addition, it is an object of the present invention to assure that a correct determination of the relative speed of the two movable parts is possible.
Because a differential position value is formed according to the invention after the absolute relationship is established, it is assured that at this moment as well, an unequivocal determination of the relative speed of the two movable parts is possible. This assures precise speed control by the evaluation unit.
Regarding the formation of the differential position value, there are various possibilities, for example, the differential position value may already be formed by the position measuring system and transmitted together with the absolute position data transmitted for the first time.
Alternatively, it is also possible to transmit both the incremental position data and the absolute position data to the evaluation unit after the absolute relationship is established and for the formation of the differential position value to take place first in the evaluation unit.
It is understood that the present invention can be used in conjunction with various position measuring systems. For example, various physical scanning principles can be employed, such as optical, magnetic, inductive, or capacitive. Both rotary position measuring systems and linear position measuring systems can also be designed in accordance with the present invention.
Further advantages and details of the present invention will become apparent from the following detailed description of preferred embodiments in conjunction with the accompanying drawings.